The present invention relates to a screw compressor for air or gas, in particular for use in pressure applications (e.g. in the conveyance of granulates or powders, or in water treatment) and in vacuum applications (e.g. in gas, fume or steam exhaust systems).
As is well known, a screw compressor comprises at least one male rotor and at least one female rotor that mesh together during rotation around respective axes and are housed inside a casing body. Each of the two rotors has screw-shaped ribs that mesh with corresponding screw-shaped grooves of the other rotor. Both the male and female rotor show, in cross section, a predetermined number of lobes (or teeth) corresponding to their ribs and of valleys corresponding to their grooves. The number of lobes of the male rotor may be different from the number of lobes of the female rotor. Already in the 1970s, the symmetrical profiles of the lobes and valleys of rotors were replaced by asymmetrical profiles in order to improve the volumetric efficiency of the screw compressors.
As in all volumetric compressors, the volumetric efficiency of the screw compressor depends on the clearance between the two rotors and between the rotors and the body encasing them (formed by two cylinders connected together). Furthermore, the volumetric efficiency of the screw compressor is influenced by the opening present between the cusp of the casing body and the head of the two rotors when they start to mesh. Through the opening, the gas contained between the valleys of the rotors is placed in communication with the intake area of the compressor; hence the gas flows back toward the latter and the volumetric efficiency declines. In cross section, corresponding to this opening there is a blow hole area having the shape of a triangle with curvilinear sides formed by the tip portions of the lobes of the two rotors. The blow hole area must be minimised by means of an accurate design of the profiles of the rotors such as to maximise the volumetric efficiency.
Starting from the definition of the profile of one of the two rotors (e.g. the female rotor) and applying the principle of “conjugate profiles”, drawn from the theory of meshing and gearings, it is possible to obtain the profile of the other rotor (in this case the male rotor). It should be noted, for the sake of completeness, that the two profiles are conjugate if and only if one profile envelops the various positions that the other profile assumes in the relative motion defined by the two polars (in the specific case of rotors, the polars are circumferences). The application of the principle of conjugate profiles to generate the rotors of a screw compressor is described, for example, in document U.S. Pat. No. 5,454,701.
Another possibility for generating the profiles of two rotors involves the use of the same generating rack, as is shown, for example, in documents WO97/43550, U.S. Pat. No. 4,643,654 and GB2418455. By rolling, without sliding, the polar of the profile of the generating rack respectively on the polar of the male rotor and on the polar of the female rotor, the profiles of the two rotors are determined as the envelope of the positions assumed by the rack profile itself.
One the problems to be confronted when designing the profiles of screw compressor rotors regards the definition of their profiles by means of cutting tools, which tend to wear easily. In particular, the construction of the female rotor is particularly critical, since the reduced thickness of its lobes limits the stresses allowable during cutting of the lobes themselves.
In this context, the technical task at the basis of the present invention is to propose a screw compressor which overcomes the limitations of the above-mentioned prior art.